In the past, aluminum alloy welded components have been used in the manufacture of vehicular wheels and housings due to their light weight properties. For example, in the manufacture of vehicular wheels as shown in FIGS. 1 and 2, a plate-shaped aluminum alloy member 11 is bent into a cylindrical shape, and then end portions 12 thereof are butted against each other and integrally welded by friction stir welding 13 therebetween, to make a cylindrical component 14. In addition, both ends are flared to make the wheel. Furthermore, to improve the corrosion resistance and the wear resistance on the design surface, a coating is formed by anodizing the design surface, which is the surface on the opposite side of the welded section.
In addition, in the case of housings 1 as shown in FIG. 3, a side member 3 and a cover member 2, which are both made from an aluminum alloy material, are integrally welded by friction stir welding to form a joint 5. The integrally-welded side member 3 and the cover member 2 are face milled to smoothen the outer surface of the welded side, including the welded section 4, and then a coating 6 is formed on the design surface by anodizing to improve corrosion and wear resistance properties.
However, color tone variations may occur in the coating 6 formed on the design surface due to differences of the crystal grain sizes between the portions corresponding to the welded section 4 and other non-welded portions.
Therefore, to eliminate color tone variations due to differences of crystal grain sizes between the portion corresponding to the welded section in the coating of the aluminum alloy welded component and other portions, it has been proposed to equalize the crystal grain sizes by performing a heat treatment prior to the anodizing treatment.
However, even if the crystal grain sizes are equalized by performing a heat treatment prior to anodizing, the color tone variations, which exist between the portion corresponding to the welded section in the coating of the aluminum alloy welded component and the other portions, may not be sufficiently reduced.